The life cycle of the fuel that is used in a nuclear reactor (referred to as the “fuel cycle”) can fall broadly into one of two categories:

  • A once-through fuel cycle, in which the used fuel exiting the reactor is destined for permanent disposal. The used fuel is removed from the reactor after achieving design burn-up and only a small fraction of the energy potentially available in the fuel is obtained.

  • A closed fuel cycle, in which more energy is extracted from the used fuel by processing it to separate the uranium and plutonium for reuse and to remove fission products.14 The other transuranics15 may also be reused or disposed of with the fission products.

The vast majority of nuclear-generated electricity in the world is produced using a once-through fuel cycle. The United States currently uses a once-through uranium fuel cycle; in this annex the committee focuses on alternatives to this fuel cycle.

Types of Closed Fuel Cycles

Closed fuel cycles fall into two major categories: (1) fuel cycles designed to produce at least as much new fissionable material as is destroyed in producing energy (“breeding fuel cycles”); and (2) fuel cycles designed to reduce the quantity of high-level nuclear waste ultimately requiring geologic disposal (“burning fuel cycles”). In either case, the used fuel is recycled, requiring chemical or electrochemical processing to separate the fissionable parts of the used fuel and new fuel to be fabricated. The new fuel is then inserted into another reactor for additional power generation. These steps have to be repeated a number of times to achieve


“Fission products” are isotopes produced as a result of the fission of a massive atom such as U-235.


“Transuranic elements” (also known as “transuranics” or “TRU”) are elements with an atomic number greater than uranium—that is, having nuclei containing more than 92 protons. Examples of transuranics are neptunium (atomic number 93), plutonium (94), and americium (95). The most important transuranic isotopes in used nuclear fuel are Np-237, Pu-239, Pu-240, Pu-241, Am-241, Am-243, Cm-242 through Cm-248, and Cf-249 through Cf-252.

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